[meteorite-list] Article - It Came from Outer Space?

From: ken newton <magellon_at_meteoritecentral.com>
Date: Fri Oct 8 23:51:27 2004
Message-ID: <4167603A.9010809_at_earthlink.net>

http://www.americanscientist.org/template/AssetDetail/assetid/37224;_QqdrDyc#37436


American Scientist Online
SCIENCE OBSERVER November-December 2004
It Came from Outer Space?
David Schneider

In 1981, I was a beginning graduate student taking a course in field
geology at UC Berkeley. This was only a year after Berkeley physicist
Luis Alvarez, his geologist son Walter and two colleagues published what
was then a startling (and not-much-believed) theory suggesting that the
impact of an asteroid or comet caused, among other things, the
extinction of the dinosaurs. If these Berkeley luminaries could offer up
such patently absurd ideas, the students figured we were entitled to do
the same. So whenever our professor queried us about some puzzling
geologic structure, we had a ready response: "Must have been an
asteroid." Alvarez's theory ultimately triumphed, and appreciation of
the importance of impact events grew enormously within the geological
community. Now earth scientists are far more ready to accept the
validity of extraterrestrial influences. But a recent episode suggests
that the pendulum might have swung too far.

In 2002, Jens Orm?, Angelo P. Rossi and Goro Komatsu, working at the
International Research School of Planetary Sciences in Pescara, Italy,
reported evidence for what they claimed was a relatively recent
meteorite strike: a field of craters located in the Abruzzi Apennines,
roughly 100 kilometers east of Rome. The largest feature of the field is
a 100-meter-diameter circular basin, situated in the Prato del Sirente
plain, close to the town of Secinaro. Associated with the main basin are
17 nearly circular depressions, which presumably formed at the same time
because the extraterrestrial object responsible for them broke up in the
atmosphere just before hitting.

Orm?'s team was unable to locate any definitive markers of an impact,
such as meteoritic material emplaced below a crater or telltale grains
of shocked quartz in the target rock. But these signs could be missing
for good reason: Quartz is almost absent from the limestone-rich
sediments found in the area, and perhaps the group's 4.5-meter-deep
excavation of one of the craters had been too shallow to reach the
meteorite they believed to be buried below. Orm? and his colleagues did
find one line of evidence that they found very compelling?curious
magnetic anomalies associated with many of the smaller craters, which
they interpreted to mean that remnants of meteorites (which are quite
often highly magnetic) were indeed buried there.

In 2003, Orm? and his two coworkers, joined by Roberto Santilli, used
radiocarbon dating to argue that the meteorite that formed this crater
field might have done more than just that, publishing their ideas in the
journal Antiquity. Their finding that the impact took place in the 4th
or 5th century A.D. fit well with a locally preserved legend that
describes people seeing a star falling to earth, an event that was
seemingly important to their conversion from paganism to Christianity.
These authors also proposed ties with the conversion of Emperor
Constantine himself, which took place at very roughly the same time and
place and was said to have been preceded by notable celestial phenomena.

Not surprisingly, this intriguing story garnered the attention of the
popular press. For example, last year New Scientist published a piece
entitled "Crater find backs falling star legend." It seems the glib
answer I gave to my geology professor a quarter-century ago had become
mainstream.

My graduate student career was not long enough to see this shift in the
attitude of the scientific community through, but it was long enough to
introduce me to Pierre Rochette, a French rock magnetist who later
became a close friend. So I was quite interested to learn that earlier
this year he and two Italian colleagues, Fabio Speranza and Leonardo
Sagnotti, published a challenge to the notion that the circular
depressions on the Sirente plain are impact craters at all, much less
ones that have anything to do with Constantine's conversion to
Christianity. (I should note that, having personal connections with one
of the players in this debate, I harbored some bias toward his position
from the outset.)

Rochette, who normally works out of the University of Aix-Marseille,
became interested in the topic while on a sabbatical at the Istituto
Nazionale di Geofisica e Volcanologia in Rome, where he discovered that
one of his new colleagues was very skeptical of the crater theory.
Speranza, a structural geologist, explains the source of his initial
doubt: "I have a house about 10 kilometers away. I've known this place
since I was a child," adding, "I knew that the landscape of Abruzzi was
full of similar shapes." Could they all be impact craters? Surely not,
he thought.

Speranza points out another difficulty with the impact-origins theory.
Large blocks of limestone sit within the boundaries of the Sirente
"crater." Such limestone would not have survived an impact. So if Orm?'s
theory is correct, one must surmise that somebody set these giant chunks
of rock in place since the crater formed. To Speranza, that just didn't
make sense. Speranza and colleagues further argue that Orm?'s
radiocarbon dating gave one age for the main feature (placing it in the
4th or 5th century a.d.) and a completely different age for a nearby
"crater" called C9, a date in the 3rd millennium B.C.

Indeed, to Speranza, the only suggestive evidence for an impact origin
seemed to be the magnetic anomalies that Orm? and his colleagues had
measured over some of the smaller depressions. But according to
Rochette, even those anomalies are easy enough to understand. One needs
simply to realize that these pockets are "dolines," places where the
limestone has dissolved and the hole has filled in with sediments that
are slightly more magnetic. Careful measurements of the magnetic
properties of these materials showed that this mechanism is sufficient
to account for the magnetic anomalies.

If not an impact crater, what is the large circular depression found in
the Sirente plain? Speranza, Sagnotti and Rochette give a plausible
answer: It is a reservoir made by human hands for the purpose of
watering herds of sheep. They describe how this area of Italy was one of
the main wool-producing regions of Europe between the 12th and 16th
centuries, although locals have been involved in the activity since
before Roman times. The great permeability of the underlying rock,
however, does not allow rivers or even large springs to form, which
creates a problem for shepherds trying to maintain millions of sheep
there through the summer. The logical answer to this problem, they
posit, was to dig reservoirs at the low points of these plains, where
water tends to accumulate.

http://www.americanscientist.org/content/AMSCI/AMSCI/Image/MediumImage_200492910720_846.jpg

Orm?, Rossi and Komatsu have refused to be questioned about this recent
challenge to their theory. In a written reply to my request for an
interview, Orm? states: "It is not possible for us to comment in [the]
media on the work done by other scientists and on our own unpublished
results." Fair enough. Curiously, the short written remarks these
authors shared with me appear far more tentative than the statements
given in their published papers. They say: "As long as the structure is
not a proven impact crater field, it is impossible to draw any
conclusions about historical consequences." This tone is in stark
contrast to almost the entire body of their Antiquity paper, which is
all about linking the structures seen in the Sirente plain with
historical events.

Speranza notes that officials in the nearby town of Secarino are now in
a bit of a quandary. After Orm?'s papers were published, they began
promoting the site as a crater park, hoping to make it a local tourist
attraction. In August of last year they held a meeting on the "crater,"
which brought together many of the local dignitaries. But now that a
significant scientific challenge has been published, it is hard to see
how officials of the community of Secarino can in good conscience go
ahead with those plans. After all, what tourist would want to visit a
"Crater or Just-Plain-Watering-Hole Park"??David Schneider


 
Received on Fri 08 Oct 2004 11:51:22 PM PDT